The present invention relates to a waveguide window construction and more particularly to such a window construction which will provide a reliable hermetic seal in waveguide systems employing a housing constructed of a material having a high coefficient of thermal expansion such as aluminum.
In some waveguide systems, it is necessary that the waveguide cavity be filled with a selected gas under pressure. In earthbound systems, a source of gas under pressure is normally utilized so that small leaks can be tolerated and thus non-hermetic or O-ring type seals may be used. However, in the compact microwave systems typically used in military weapons and in space systems, the waveguide systems must be hermetically sealed since the space, weight and power penalties for an active pressurization are far too great. Microwave signals are typically coupled into and out of waveguide cavities through windows of a glass or glassy material which can transmit the electromagnetic energy without significant attenuation.
The waveguide window is typically provided with a metal frame having a coefficient of thermal expansion (CTE) which is matched to the glass, e.g. KOVAR. Other, functionally equivalent alloys are known in the art. As is understood by those skilled in the art, the glass materials typically used for microwave windows, e.g. Corning 7052, have an expansion rate of approximately 5.times.10.sup.-6 ppm/.degree. C. and the KOVAR alloy essentially matches that expansion. Corning 7070 with a CTE of about 3 may also be used. Attachment of the window to the frame is accomplished by direct glass flow to an oxide layer on the frame or by a glass metalization process which allows the glass to be subsequently soldered to the KOVAR frame, the KOVAR frame also being plated to facilitate the solder attachment.
In prior art implementations of waveguide windows, the frame assembly is typically soldered to the waveguide housing or assembly. The solder joints on these prior art designs are typically thin and of a design which places the solder joint in shear under thermal cycling conditions. However, when the waveguide housing is constructed of aluminum, difficulties have been encountered due to the extremely high strains which are generated by differential expansion when the assembly is subjected to thermal cycling. If a hard solder is used, such as 80/20 Au/Sn, the strains may be passed to the glass which can fracture. If soft solder is used, the thermal cycling may cause solder fissuring and a loss of hermeticity. The origins of the strains are thus similar to those occurring in hermetically sealed feedthroughs which were the subject of my previously issued U.S. Pat. No. 4,841,101.
In accordance with the practice of the '101 patent, strains were reduced to a manageable level by providing a thicker and more uniform layer of solder around a feedthrough ferrule, e.g. a thickness defined by means of a flange on the inner end of the feedthrough ferrule. Waveguide windows, however, are typically much larger than electrical feedthroughs so that the strains produced by differential thermal expansion are substantially larger, preventing the straightforward application of the technique taught in the '101 patent.
Among the several objects of the present invention may be noted the provision of a novel waveguide window construction; the provision of such a construction which facilitates the maintenance of a hermetic seal; the provision of such a construction which may be employed with waveguide housings of a metal having a high coefficient of thermal expansion; the provision of an assembly which is highly reliable and which is of relatively simple and inexpensive construction. Other objects and features will be in part apparent and in part pointed out hereinafter.